How to Correctly Choose a Surge Protector(SPDs).

How many categories can surge protectors be divided into?

Type 1 (Voltage Switching Type)

• Function:
  Primarily used to discharge large currents (10/350μs waveform) from direct or induced lightning strikes.
  Typically applied at the main power distribution entrance of buildings (transition zone from LPZ0 to LPZ1).
• Working Principle:

High impedance in normal state: 

When there is no surge, the SPD remains in a high-impedance state, not affecting the circuit.
Low impedance upon triggering:

When the surge voltage exceeds the threshold (e.g., 4kV), the internal gas discharge tube (GDT) or spark gap breaks down, forming a low-impedance path to instantly discharge currents of tens of kA.
Subsequent arc extinguishing: After the surge dissipates, the GDT restores to a high-impedance state through its self-recovery.

• Core Components：

Gas Discharge Tube (GDT): Conducts through ionization of inert gas.
Spark Gap: Discharges through air breakdown, with strong current-handling capacity (can exceed 100kA).

Type 2 (Voltage Limiting Type)

• Function:

Protects against induced lightning and switching overvoltages (8/20μs waveform).

Used in distribution panels.

• Working Principle:

Nonlinear impedance characteristic: 

When the surge voltage rises, the internal varistor (MOV) impedance drops sharply, clamping the voltage to a safe level (e.g., Up ≤ 1.5kV).

Continuous discharge: 

The MOV can repeatedly discharge moderate currents (20–40kA), but high currents may degrade its performance.

• Core Component:

Metal Oxide Varistor (MOV): Composed of zinc oxide (ZnO) particles, a voltage-sensitive semiconductor device.

Type 3 (Combined or Fine Protection)

• Function

Fast response: Utilizes TVS diodes or MOV+TVS combinations with a response time ≤1ns.

Ultra-low clamping voltage (e.g., Up ≤ 0.8kV), ensuring the safety of sensitive equipment.

Core component:

Transient Voltage Suppression (TVS) diode: Leverages PN junction avalanche effect for extremely fast response but has limited current-handling capacity (<5kA).

Choosing the right surge protective device (SPD) is crucial. How should you select one for different scenarios?

1. Define Protection Requirements

1.1 Identify Surge Sources

Direct lightning strike (e.g., building hit):  Requires Type 1 SPD (10/350μs waveform).

Induced lightning or switching overvoltages (e.g., grid fluctuations, equipment startup/shutdown): Requires Type 2 or Type 3 SPD (8/20μs waveform).

1.2 Determine Protection Zones (LPZ)

LPZ0 → LPZ1 zone (e.g., main power intake):

 Type 1 or Type 1+2 hybrid SPD.

LPZ1 → LPZ2 zone (e.g., floor distribution panel):

Type 2 SPD.

LPZ2 → LPZ3 zone (e.g., equipment front-end):

Type 3 or precision SPD.

2.Key Parameter Selection

2.1 Maximum Continuous Operating Voltage (Uc)

2.1.1 Must be higher than the system’s rated voltage (e.g., for a 385V system, select Uc ≥ 385V).

2.2.2 Account for grid fluctuations (±10~20%).

2.2 Voltage Protection Level (Up)

2.2.1 Lower Up provides better protection but must match the protected equipment’s withstand voltage.

2.2.2 General rule: Up ≤ 80% of the equipment’s withstand voltage (e.g., if equipment can handle 2.5kV, choose Up ≤ 2.0kV).

2.3 Current Handling Capacity (In / Imax)

• Type 1: In ≥ 12.5kA (10/350μs waveform).
• Type 2:Main distribution panel: Imax ≥ 40kA (8/20μs).

Sub-distribution panel: Imax ≥ 20kA (8/20μs).

• Type 3: In ≥ 5kA (combination wave).

2.4 Response Time

Standard SPDs: ≤25ns.

For precision equipment, choose faster options (e.g., TVS diodes, ≤1ns).

3. Selection by Application Scenario

3.1 Power Supply Systems

3.1.1 Main Distribution Panel: Type 1+2 hybrid SPD (e.g., Imax = 100kA, Up ≤ 2.5kV).

3.1.2 Sub-Distribution Panel: Type 2 SPD (e.g., Imax = 40kA, Up ≤ 1.8kV).

3.1.3 Equipment End: Type 3 or socket-mounted SPD (e.g., Up ≤ 1.2kV).

3.2 Signal/Communication Lines

3.2.1 Use dedicated signal SPDs, matching interface types (e.g., RJ45, RS485).

Pay attention to the transmission rate and insertion loss (if you are using a Gigabit network, choose a high-frequency compatible model)

3.3 PV/DC Systems

Select DC SPDs with Uc ≥ 1.2 × system max voltage.